Indeed in German law a partially void contract becomes completely void unless it is protected by a severability clause (which means in practice all contracts have them and they usually state that the void part is to be replaced by something with the same or similar economic consequences).

Standard form contracts (take it or leave it rules that are distributed to everyone) are exempt from this though (here the void part falls back to default rules).

Indeed, the normal threshold for whether a contract was/is an AGB (stnadard form contract; allgemeine geschaeftsbedingungen = general rules of business ~ TOS) is whether it was practically negotiable.

No, that effectively enables scams. To prevent scams courts disallow using inability to enforce unlawful clause as a reason to terminate the contract. You typically buy goods, not contracts.